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Laser-Plasma Accelerators as Sources of Electron Beams at 1 MeV to 1 GeV Eric Esarey FLS Workshop, March 1 -5, 2010 http://loasis.lbl.gov/ W. Leemans, C. Geddes, C. Schroeder, S. Toth, A.Gonsalzas, J. van Tilborg, K. Nakamura, M. Chen and others LOASIS Program Lawrence Berkeley National Laboratory

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Laser-plasma accelerators: Outline Self-modulated LWFAs: Status Prior to 2004 LWFAs: High quality e-beam production at 100 MeV-level (2004) LWFAs: High quality e-beam production at 1 GeV-level (2006) Downramp injection at 1 MeV-level (2008) Integrated gas jet+capillary structure (2009) Colliding pulse injection at 100 MeV-level (2006, 2009) Ionization injection at 100 MeV-level (2008, 2010)

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Laser Wakefield Accelerator (LWFA) B.A. Shadwick et al., IEEE PS. 2002 Standard regime (LWFA): pulse duration matches plasma period Ultrahigh axial electric fields E z > 10 GV/m, fast waves Ultrashort plasma wavelength p ~ 30 m (100 fs) Plasma channel: Guides laser pulse and supports plasma wave Tajima, Dawson (79); Gorbunov, Kirsanov (87); Sprangle, Esarey et al. (88)

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Basic design of a laser-plasma accelerator: single-stage limited by laser energy laser E z wake Laser pulse length determined by plasma density k p z 1, z ~ p ~ n -1/2 Wakefield regime determined by laser intensity Linear (a 0 1) Determines bunch parameters via beam loading Ex: a 0 = 1 for I 0 = 2x10 18 W/cm 2 and 0 = 0.8 m Accelerating field determined by density and laser intensity E z ~ (a 0 2 /4)(1+a 0 2 /2) -1/2 n 1/2 ~ 10 GV/m Energy gain determined by laser energy via depletion* Laser: Present CPA technology 10s J/pulse * Shadwick, Schroeder, Esarey, Phys. Plasmas (2009)

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State-of-the-Art Prior to 2004: Self-Modulated Laser Wakefield Accelerator (SM-LWFA) Self-modulated regime: Laser pulse duration > plasma period Laser power > critical power for self-guiding High-phase velocity plasma waves by Raman forward scattering Self-modulation instability laser pulse Plasma density wave Sprangle et al. (92); Antonsen, Mora (92); Andreev et al. (92); Esarey et al. (94); Mori et al. (94) SM-LWFA experiments routinely produce electrons with: 1-100 MeV (100% energy spread), multi-nC, ~100 fs, ~10 mrad divergence Modena et al. (95); Nakajima et al. (95); Umstadter et al. (96); Ting et al. (97); Gahn et al. (99); Leemans et al. (01); Malka et al. (01) Gas jet Laser beam Parabolic mirror Mirror CCD e - beam -40-2002040 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 Detection Threshold Energy Distribution Red: 1000 psi He Blue: 500 psi He Leemans et al. (02) Few TW 10 20 cm -3 nCs

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30 Sep 2004 issue of nature : Three groups report production of high quality e-bunches Approach 1: Plasma channel LBNL/USA: Geddes et al. Plasma Channel: 1-4x10 19 cm -3 Laser: 8-9 TW, 8.5 m, 55 fs E-bunch: 2 10 9 (0.3 nC), 86 MeV, E/E=1-2%, 3 mrad Approach 2: No channel, larger spot size RAL/IC/UK: Mangles et al. No Channel: 2 10 19 cm -3 Laser: 12 TW, 40 fs, 0.5 J, 2.5 10 18 W/cm 2, 25 m E-bunch: 1.4 10 8 (22 pC), 70 MeV, E/E=3%, 87 mrad LOA/France: Faure et al. No Channel: 0.5-2x10 19 cm -3 Laser: 30 TW, 30 fs, 1 J, 18 m E-bunch: 3 10 9 (0.5 nC), 170 MeV, E/E=24%,10 mrad Channel allows higher e-energy with lower laser power Breakthrough Results: High Quality Bunches

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GeV: channeling over cm-scale Increasing beam energy requires increased dephasing length and power: Scalings indicate cm-scale channel at ~ 10 18 cm -3 and ~50 TW laser for GeV Laser heated plasma channel formation is inefficient at low density Use capillary plasma channels for cm-scale, low density plasma channels Capillary 3 cm e - beam 1 GeV Laser: 40-100 TW, 40 fs 10 Hz Plasma channel technology: Capillary

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GeV Beams in 3cm 40TW laser Capillary discharge 1 Tesla magnetic spectrometer Optical diagnostics (not shown) Divergence(rms): 1.6 mrad Energy spread (rms): 2.5% Resolution: 2.4% 3cm Leemans et al., Nature Physics 2006

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Wake Evolution and Dephasing Yield Low Energy Spread Beams in PIC Simulations WAKE FORMING INJECTION DEPHASING Propagation Distance Longitudinal Momentum 200 0 Propagation Distance Longitudinal Momentum 200 0 Propagation Distance Longitudinal Momentum 200 0 Geddes et al., Nature (2004) & Phys. Plasmas (2005)

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LWFA: Production of a Monoenergetic Beam 1.Excitation of wake (e.g., self-modulation of laser) 2.Onset of self-trapping (e.g., wavebreaking) 3.Termination of trapping (e.g., beam loading) 4.Acceleration If > dephasing length: large energy spread If dephasing length: monoenergetic Wake Excitation TrappingAcceleration: L accel ~L dephase 142-3 z-v g t vv Momentum Phase Dephasing distance :

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GeV Beams Repeatable but not Stable Available Controls not Sufficient Accelerator performance Laser energy, pulse width, plasma density, discharge delay, plasma channel density, depth, and length, degree of ionization But optimizing injection does not optimize guiding (accelerating structure) Need to separate injection and acceleration

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Reducing energy spread and emittance requires controlled injection Self-injection experiments have been in bubble regime: Cannot tune injection and acceleration separately Emittance degraded due to off-axis injection and high transverse fields. Energy spread degraded due to lack of control over trapping Use injector based on controlled trapping at lower wake amplitude and separately tunable acceleration stage to reduce emittance and energy spread Y[m] X[m] 5 -5 8002000 Transverse motion

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Basic physics of downramp injection Bucket length ~ 1/n Phase velocity drop enables trapping 13 nene x10 18 5 1 z [mm] 0 -0.5 0.51.030.0 Laser

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Down-ramp Injector Demonstrated: Simulations Show Injector Coupled to Low Density Accelerator Produces Low Energy Spread Beams Inject low E: E conserved during acceleration so as E , E/E Geddes et al., PRL V 100, 215004 (2008)*Nieter et al., JCP 2004 Accelerator: 3 - 50 cm; n~10 17 -10 18 cm -3 n z Plasma ramp injector: 1mm; 10 19 cm -3 laser Laser 10TW e-e- Jet Laser focused on down-ramp of gas jet density profile MeV beam produced with Low divergence (20 mrad) Good stability Central energy (760keV/c 20keV/c rms) Momentum spread (170 keV/c 20keV/c rms) Beam pointing (1.5 mrad rms) Laser transmission 70% and mode still good for driving wakefield Energy Spectrum at Ramp Exit #/P z (MeV/c) P 1.5MeV/c P 200keV/c Energy Spectrum at 3mm P z (MeV/c) P 20MeV/c P 200keV/c

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Gas Jet Nozzle Machined Into Capillary Can Provide Local Density Perturbation laser e - beam 1mm Laser-machined gas jet Axis of the capillary 0.2mm Measured surface profile Density profile in jet region

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Jet Improves Beam Stability Input Parameters: P jet 145psi, N e 2x10 18 cm -3,a 0 1 (25TW), Laser pulse length 45 fs laser NB: Both data sets show subsequent shots Pointing 0.8 mrad Divergence 1mrad Energy 300MeV 7MeV E/E 6% 0.7% Q 7.3pC 1.7pC Stability with jet Best stability without jet Pointing 1.8 mrad Divergence 1mrad Energy 440MeV 95MeV E/E 4% 2% Q 2.6pC 2.0pC Input Parameters: no jet in cap, N e 2x10 18 cm -3,a 0 1 (25TW), Laser pulse length 45 fs

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Colliding pulse allows control of injection -1.5 -0.5 0 0.5 1.0 1.5 2.0 Untrapped Wake Orbit Trapped + Focused Wake Orbit Beat Wave Separatrices -20 1 23 Phase Space Add two counter-propagating laser pulses Collision produces laser beat wave with slow phase velocity 3-pulse colliding pulse [Esarey et al. PRL (1997), Schroeder et al., PRE (1999)] 1. control of injection position: delay between pump and trailing pulses 2. control injected charge: laser intensities and pulse durations 3. control beat phase velocity: different laser frequencies 2-pulse version: Pump + backward [Fubiani et al., PRE (2004)] kpzkpz 3-pulse Colliding Pulse Injection

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k p (z-ct) laser trapped orbits untrapped orbits laser Controlled injection via colliding laser pulses improves beam quality Esarey et al. PRL (1997); Schroeder et al. PRE (1999); Fubiani et al. PRE (2004); e-e- Leemans et al. AAC (2002); (2004); Faure et al. Nature (2006); Rechatin et al. PRL (2009); Kotaki et al. PRL (2009) Experiment: laser a=0.35 a=1.2 Gas jet: 7x10 18 cm -3 Pump laser (drives wake) Colliding laser pulse 3 mm Rechatin et al. Phys. Rev. Lett. (2009) LOA (France): Faure et al., Nature (2006) Experimental demonstration (2-pulse): 1% FWHM energy spread Theory:

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Colliding pulse experiments at LBNL Colliding pulse experimental setup online Experimental plan: Step1: demonstrate reliable injector Step 2: accelerator/laser control for high energy Step 3: tune for high quality beam 12 TW system

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Colliding pulses produce stable, reproducible beam Scan timing of collider Charge measured on phosphor screen, ICT Timing window as expected from simulation ~20% rms charge stability Q ICT ~ O[40pC] Phosphor Charge vs. collision timing e-beam image Simulation at a=0.5 Geddes et al., ongoing

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Ionization-induced trapping using high-Z gas (nitrogen) laser z Injection region,

Laser-plasma accelerators: Summary Self-modulated LWFAs: Status Prior to 2004 100% energy spread, max energy > 100 MeV, nCs of charge LWFAs: High quality e-beam production at 100 MeV-level (2004) Narrow energy spread, small divergence, 100 MeV, 100s pC LWFAs: High quality e-beam production at 1 GeV-level (2006) Narrow energy spread (few %), small divergence (few mrad), 1 GeV, 10s pC Few-cm long plasma channel guiding (capillary discharge) Downramp injection at 1 MeV-level (2008) Good stability, narrow absolute momentum spread (170 keV/c), 100s pC Integrated gas jet+capillary structure (2009) Improved stability, few % energy spread, 0.5 GeV, few pC (ongoing) Colliding pulse injection at 100 MeV-level (2006, 2009) Good stability, narrow energy spread (1%), 180 MeV, 10 pC Ionization injection at 100 MeV-level (2008, 2010)